Thin polished plates such as silicon wafers and the like are a very important part of modern technology. A wafer, for instance, may refer to a thin slice of semiconductor material used in the fabrication of integrated circuits and other devices. Other examples of thin polished plates may include magnetic disc substrates, gauge blocks and the like. While the technique described here refers mainly to wafers, it is to be understood that the technique also is applicable to other types of polished plates as well. The term wafer and the term thin polished plate may be used interchangeably in the present disclosure.
Fabricating semiconductor devices typically includes processing a substrate such as a semiconductor wafer using a number of semiconductor fabrication processes. Lithography, for example, is a process that can be used to pattern parts (or the entirety) of a wafer. Lithography may be carried out using projection exposure systems such as scanners or the like.
Typically, the wafer is aligned and a focus leveling map is determined prior to the exposure. The focus leveling information may contain small perturbations (may also be referred to as focus spots) that are caused by various sources of front side and backside contamination. In addition to causing imaging issues during the exposure, these focus spots can impact alignment and, as a result, overlay of the current exposure layer compared to the previous layer. The reason for this is that a focus spot can cause local deformation of the wafer shape in the area immediately on or around the alignment mark, causing the tool to measure the alignment mark location at a slightly different location than it would have measured had no local deformation been present. As a result, this measured location data is used in the regression analysis and determination of the alignment model terms. This information is then applied to the scanner stage during exposure to ensure proper positioning between a reticle, lens, and wafer and therefore, better overlay. When these correction values are calculated incorrectly or sub-optimally, loss of overlay control and reduction in overlay accuracy occur as the result.
Therein lies a need for providing effective and accurate alignment methods and systems.